Process for producing a plastic casing reinforced against tensile force

ABSTRACT

A process for forming a plastic casing having reinforcing elements to resist tensile forces by simultaneously extruding the plastic casing and the reinforcing elements which are in a molten form and then twisting the extruded casing and elements to impart a helical configuration to the elements.

BACKGROUND OF THE INVENTION

The invention relates to a process for producing a plastic casingreinforced against tensile forces by, elements for resisting extensionin the plastic casing. The process includes the elements being extrudedin molten form at the same time as the plastic casing serving to receivethese elements is extruded.

In many cases in which plastic is used as a casing or walling material,the strength value of most plastics does not satisfy the demands made onthe casing. Therefore, it is known, for example for plastic insulationsfor electrical and/or optical cables or lines, to incorporate in theinsulation or to bond in a suitable form with the insulation elementsfor resisting extension (German Patent 2,740,162, German Patent2,818,297). In such cases, the elements for resisting extension havebeen placed on or under the insulation and, for example, are adhesivelybonded with the insulation, jacket or casing. Processes are also knownin which the elements for resisting extension have been fed to theinsulation during extrusion.

JP-A-62,130,821 discloses a process in which a tubular body is extruded,reinforcing filaments which may consist, for example, of LCPs and arealso embedded in the tube walling being extruded in coextrusion by meansof tubular dies. These reinforcing filaments run in straight lines,which may be disadvantageous for some applications, for example in thecase of strong bending stresses, in the case of stranding operations,etc.

Also to be cited in this connection as prior art are U.S. Pat. Nos.4,553,815 and 4,767,183, in which although the use of LCPs reinforcingagainst tensile forces is already described in the case of opticalcables, their simultaneous extrusion is not.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a jacket, casing orcovering which is particularly suitable for optical glass fibers and canbe used universally. The way in which this object is achieved accordingto the invention in the case of a process of the type mentioned at thebeginning is that the plastic casing serves for producing coverings foroptical glass fibers or cables and that, after extrusion, the plasticcasing is subjected to a torsional operation before or during thewinding-up process, in such a way that the elements for resistingextension run spirally.

In this way, the elements for resisting extension distribute themselveshelically, seen in the longitudinal direction, about the longitudinalaxis of the covering and, as a result, the latter can optimallywithstand, for example, bending and/or stranding operations.Consequently, an optimum protection of the sensitive optical waveguidesagainst mechanical stresses is ensured. The casing and the elements forresisting extension are produced in the desired arrangement from oneextrusion apparatus in one process. To be regarded as the specialfeature in any event is that the reinforcing filaments are incorporatedin the desired form in the plastic casing in the molten state of thematrix. Consequently, the primary casing thus produced already fulfillsa reinforcing function, which in some cases allows dispensing withadditional or further reinforcing elements (Kevlar, GRP, wirereinforcement). This form of reinforcement also allows tough or flexibleplastics having good bending properties in a broad temperature range tobe used for the primary casings.

In a development of the invention, an embodiment is preferred in whichthe elements for resisting extension consist of polymers based on liquidcrystal polymers (LCPs). This achieves the effect that the elements forresisting extension can be extruded particularly simply in molten format the same time as the plastic casing or profile serving to receivethese elements.

The torsional operation can be accomplished by using stranding devices,in particular based on SZ-stranding devices. In the case of a stationaryextrusion die, the extrudate is twisted by making the take-off and thewind-up rotate and thus obtaining a helical fiber reinforcement, or theextrudate is turned alternately to the right and to the left between theextruder head and the wind-up. By this turning, a stranding of theelements which are introduced through the extruder head into the casingtube is also obtained at the same time.

In a further development of the invention, the extrusion die can also berotated. The advantage of this technique over the conventional insertionof prefabricated reinforcing elements is that the feeding of thereinforcing material is very simple, because there are no rotatingrun-off points. The arrangement of the reinforcing filaments can,moreover, be fixed according to requirements.

The tube or the covering becomes particularly stiff if the filamentsreinforcing against tensile forces are transferred far to the outsideand more flexible if the filaments are positioned as far as possible tothe inside. Multi-row arrangement or symmetrical and unsymmetricalgrouping of the reinforcing filaments are possible.

Suitable materials into which the stiffened filaments are embedded areboth amorphous and partially crystalline plastics, as well as polyblendsand copolymers, for example polyamide, polycarbonate, polyester,polymethane, polyethylene, polypropylene, fluoroplastics such as PFA,ETFE [sic], FEP etc. For some applications, the reinforcing filaments donot have to be firmly bonded with the polymer matrix, but merely have amore or less good frictional engagement with the matrix.

The invention relates furthermore to a process for producing a plasticcasing reinforced against tensile forces by elements for resistingextension in the plastic casing being extruded in a molten form at thesame time as the plastic casing serving to receive these elements, whichprocess is characterized in that a tube for a shrink-fit tube isextruded from crosslinkable plastic, in the wall of which the elementsfor resisting extension are incorporated and thus the plastic casingserves as a tube for a shrink-fit tube.

In the case of this further specific application, the production oftubes for shrink-fit tubes is possible. In this case, a tube is extrudedfrom crosslinkable plastic, the wall of which has a number of suchreinforcing filaments. These reinforcing filaments are of a plasticwhich, during the shaping process of the crosslinked shrink-fit tube,still has such a high tensile strength that the shrink-fit tube isexpanded only in the radial direction and retains its length in thelongitudinal direction during the expanding operation, without specialprocess engineering measures. In the case of this design, reinforcingfilaments which fuse or adhesively bond well with the material in whichthey are embedded are preferably extruded.

The invention is explained in detail by means of the illustrativeembodiments described below and represented diagrammatically in thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an optical multifiber unit,

FIG. 2 is a cross sectional view through a plastic angle profile and

FIG. 3 is a cross sectional view through a flat line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the case of the optical multifiber unit 11 represented in FIG. 1, inwhich a plurality of optical fibers 10 having an excess length of >1°/ooare arranged in a plastic casing 15, and this plastic casing is athermoplastic which is reinforced against tensile forces by theinsertion of elements for resisting extension in the form of reinforcingfilaments 16 of high tensile strength. The individual optical fibers 10are in this case embedded in a filling compound 12.

The plastic angle profile 25 represented in cross section in FIG. 2 hasinserted elements for resisting extension in the form of reinforcingfilaments 26 on both legs 27.

Finally, FIG. 3 shows in cross section a flat line 31 with opticalfibers 30 embedded in the plastic insulation or material 35. Reinforcingelements 36 are inserted in the plastic material between or next to theoptical fibers 30.

We claim:
 1. In a process for producing a plastic casing reinforcedagainst tensile forces by elements for resisting extension disposed inthe plastic casing, the process including the elements being extruded ina molten form at the same time as the plastic casing which receives theelements, the improvement comprises the plastic casing producing a tubeforming a covering for optical glass fibers or cables, said glass fiberhaving an excess length to the plastic casing, said elements forresisting extension being polymer elements and subjecting the plasticcasing and said polymer elements to a torsional operation afterextrusion and prior to completing a winding-up process so that thepolymer elements for resisting extension run helically within saidplastic casing.
 2. In a process as claimed in claim 1, wherein thepolymer elements for resisting extension consist of liquid crystalpolymers.
 3. In a process as claimed in claim 1, wherein the torsionaloperation is accomplished by using stranding devices.
 4. In a process asclaimed in claim 3, wherein the torsional operation is carried out witha stationary extrusion die.
 5. In a process as claimed in claim 3,wherein an SZ-stranding device serves as the stranding device.
 6. In aprocess as claimed in claim 1, which includes rotating the extrusiondie.
 7. In a process as claimed in claim 1, which includes embedding theoptical glass fibers in a filling compound.
 8. In a process as claimedin claim 1, wherein the plastic casing is extruded from cross-linkableplastic, and the plastic casing serves as a shrink-fit tube.
 9. In aprocess as claimed in claim 8, wherein the polymer elements forresisting extension are of a plastic which, during the shaping processof the crosslinked shrink-fit tube, still has such a high tensilestrength that the shrink-fit tube is expanded only in the radialdirection in the shaping operation.
 10. In a process as claimed in claim8 wherein the polymer elements for resisting extension are fused withthe material of the plastic casing in which they are embedded.
 11. In aprocess as claimed in claim 8 wherein the polymer elements for resistingextension are adhesively bonded with the material of the plastic casingin which they are embedded.
 12. In a process as claimed in claim 1,wherein the torsional operation is carried out with a stationaryextrusion die.